Electroless plating (or electroless deposition) of copper and other metals has received increasing interest in recent years. This interest is due in part because of the relatively low cost of electroless processes compared to other (e.g., vacuum) deposition techniques, and because of generally surface-controlled, selective, conformal deposition properties of electroless processes. Electroless deposition has a number of potential applications, such as repair of marginal seed layers for copper damascene electroplating, creation of seed layers and barrier layers directly on dielectrics that can be plated, and selective deposition of barrier and electromigration capping layers onto damascene metal (e.g., cobalt and cobalt alloys on copper).
Conventional electroless metal deposition is conducted in a system containing one or multiple open baths containing plating solution. In a typical operation, a wafer holder immerses a substrate wafer face-down in the plating solution during plating operations. The plating solution is exposed to ambient air, especially when the substrate wafer is being moved and the wafer holder does not cover the plating bath surface. Thus, an open bath system has disadvantages. For example, during the metal deposition step, ambient oxygen is readily dissolved in the solution, and the dissolved oxygen can interfere with the desired metal deposition (e.g., by slowing or preventing metal deposition). Electroless plating operations are typically performed at elevated temperatures in a range of 40° C. to 90° C., typically in a range of about 50° C. to 80° C. The plating solution components have a tendency to evaporate. The tendency of water and volatile components to evaporate is exacerbated by the need to ventilate the gaseous spaces over a plating bath, especially to remove explosive or toxic fumes inherent to the electroless solution (e.g., ammonia gas) or created by spontaneous decomposition of its components (e.g., dimethylamine, hydrogen). The heating load caused by evaporation substantially increases the size and costs of a heater required to maintain plating bath temperature. Condensation of evaporate bath constituents on plating-cell walls and on the wafer holder are a source of backside contamination.
A conventional electroless plating bath typically can have a bath volume of 20 liters or more. Typical bath turnover rates required to avoid plate-out and composition drift are 6 hours to 10 hours. Assuming a processing rate of 20 wafers per hour, approximately 160 wafers can be processed with 20 liters.
A problem of both face-down and face-up plating configurations is hydrogen-bubble entrapment on the plating surface and resulting defects. Hydrogen gas is created as a byproduct of almost all known electroless plating-solution reducing agents. A byproduct of most electroless plating oxidation half-reactions (i.e., the oxidation of the reducing agent) and of the self-degradation of the reducing agents is dissolved molecular hydrogen (H2). As these reactions continue (i.e., plating reactions and bath-aging), the amount of hydrogen increases until the solution becomes saturated and eventually supersaturated with dissolved hydrogen. When this occurs, the formation of hydrogen gas (bubbles) is spontaneous, and occurs most readily on solid interfaces (e.g., vessel walls, wafer surfaces). Areas in which bubbles are attached to the wafer are not plated, creating defects. Therefore, it is advantageous to utilize designs that minimize the propensity for hydrogen formation, or minimize the effective bath age.
Solution pH influences the reaction rate of the electroless plating process. It is often useful to utilize an alkaline pH-adjuster, for example, lithium-, sodium-, or potassium-hydroxide, but preferably ammonium- or tetramethylammonium hydroxide (“TMAH”) to maintain or adjust the pH. Alkali metal pH-adjusters are inexpensive, but are often unsuitable for semiconductor applications because of their rapid diffusion into and poisoning of various device materials. Ammonium hydroxide is also inexpensive and does not generally degrade device performance, but it is volatile. Therefore, the maintenance of ammonium hydroxide concentration in a plating bath is problematic. TMAH and other analogous organic cation hydroxides do not suffer from either of these problems, but are significantly more expensive. The constituents of a semiconductor electroless plating solution, particularly the reducing agents and TMAH, can be expensive, leading to bath costs in a range of $25/liter to $100/liter. Therefore, one would like to use lower cost materials without the negative impacts. Also, the waste treatment of electroless plating solutions is complicated and expensive. A waste treatment process generally involves forced decomposition of the reducing agents, accompanied by hydrogen gas stripping and dilution. A small amount of dissolved reducing agent can spontaneously breakdown to create a large volume of hydrogen gas in a storage container (an explosive hazard), so the stripping of reducing agents must be driven to completion. A plating solution must also be stripped of metal. The cost of such plating solution post-processing (including capital equipment costs) is typically in a range of $5/liter to $10/liter. Inefficient use of the plating solution, therefore, increases the cost of plating operations significantly.
Electroless plating solutions are also often inherently unstable. The instability manifests itself in auto-degradation of bath constituents and in the “plating-out” of bath metal as fine metallic particulate in the bulk solution and onto processing equipment walls, filters, and other system components. The presence of plate-out particles also increases the number of defects in the workpieces and diminishes process yield. Generally, the instability of plating solutions increases with reducing agent concentration and with temperature, and decreases with the addition of bath “stabilizers” (e.g., oxygen, chlorine, lead, tin, cadmium, selenium, tellurium). In opposition to this trend, the initiation of electroless plating of a particular metal onto a substrate and the plating deposition rate are also proportional to reducing agent concentration and temperature, and decrease with the addition of bath stabilizers. Thus, plating-solution instability and electroless plating rate and nucleation are inherently linked in a non-advantageous manner. While filtration can remove particles from a plating bath, the usefulness of such filtration when the plating bath is maintained at plating operating temperature is, at best, marginal. Filtration at elevated temperature only temporarily combats the effects of auto-degradation/particulation as particles remain in contact with the plating bath reactants, consume reactants, and grow. Some complex proposals for continuously removing electroless plating particles have been proposed in the prior art (e.g., a magnetic conveyer belt “filter”).
An electroless plating tool's fluid system is typically maintained at an elevated temperature to increase the propensity for reaction (deposition). However, several factors typically complicate the implementation of such electroless deposition systems. A generally accepted rule for chemical reactions at near-ambient conditions is that the chemical reaction rate doubles for each 10° C. increase in temperature (based on typical activation energies for chemical reaction rates). Due to this exponential thermal dependence, any heating system that locally over-drives the plating solution is more likely to cause adverse localized chemical reactions.
To be useful for semiconductor applications, the working electroless plating bath and wafer must be of uniform condition. As applied to thermodynamics, and to prevent wafer-to-wafer variation, the plating tool must reach a condition of equilibrium including all heated components. In a conventional system, the mass of heated components may be quite large, thereby requiring substantial time to stabilize. A large-volume bath will also require increased time to heat from ambient to process temperature (it is generally preferred to first mix/create these baths in the cold-state, hence the assumption of ambient starting condition).
For those process chemistries which are most unstable, point of use mixing has been used in the prior art to minimize the thermal exposure of the bath. In such point-of-use mixing, reactive constituents are combined in-line with the balance of the treatment solution immediately prior to use. However, such in-line mixing suffers from mixture-ratio transients, and the resultant fluid cannot readily be reclaimed and reused.
In traditional fluid heating schemes, thermal energy is typically added to the fluid by use of either radiation (e.g. infra-red) or conduction (e.g. heating element). With an IR heating system, the fluid must be contained within an object that allows the infrared energy to penetrate through the wall to the fluid itself. However, materials suitable for semiconductor processing fluids are not entirely transparent to IR energy, thereby causing the containment wall to heat up above the bulk fluid temperature. Similarly, conductive heating systems are necessarily affected by over-temperatures, since it is the difference in temperature (δT) that drives the desired heat transfer. Conductive heating requires that the heating element be operated at a temperature above the desired bulk-fluid temperature. The higher the thermal gradient, the faster the fluid can be raised to the desired temperature. As applied to the heating of electroless plating chemistries, these plating baths become increasingly reactive/unstable at higher temperatures. To ensure a uniform film on a semiconductor wafer, it is imperative that precise and uniform temperature be maintained. Hence, several shortcoming of conductive heating are evident. The need to maintain the heating element above the desired fluid temperature causes excessive bath degradation. The fluid heating rate is proportional to the heating element temperature. Therefore, in order to hasten the heating rate of the bulk-fluid, it becomes necessary to increase the heater temperature, thereby causing a small portion (proximal to the heating element) of the fluid to be over-heated (before convection equilibrates the bulk-fluid temperature). To attain fast and precise temperature control, event timing becomes critical. Therefore, if the heated fluid is not removed from the heating vessel in a timely and consistent manner, the wafer-wafer thermal environment varies (resulting in a variation in film thickness).
FIG. 1 depicts schematically a generalized conventional system 100 of the prior art for conducting electroless plating. In a system 100, a plating cell 102 contains plating bath 104 comprising electroless plating solution. A substrate wafer 106 in a substrate holder 108 is immersed face-down in plating bath 104 during metal deposition. Plating solution is recirculated from plating cell 102 to an overflow reservoir 110 containing recirculated solution 112. Recirculated solution then flows via pump 114 through a heater 116 and a filter 118, to be returned to plating cell 102. Typically, reservoir 110 is substantially closed to the atmosphere. Generally, a portion or all of the plating solution is drawn off through drain 120 for disposal, and a corresponding amount of fresh plating solution from liquid source 122 replenishes liquid in system 100. System 100 suffers from several disadvantages. The entire liquid volume included in plating cell 102 and reservoir 110 must be maintained substantially at the intended wafer-treatment temperature, resulting in significant chemical degradation and high cost of operation. Filtration and metrology must be accomplished at elevated temperatures, resulting in reduced lifetime and greater sensor variability. Also, when wafer 102 and substrate holder 108 are not in a lowered plating position as depicted in FIG. 1, a relatively large surface area of plating solution in plating cell 104 is exposed to the atmosphere. As described above, such exposure leads to constituent evaporation and plating-solution degradation. Furthermore, fluid filter 118 shares the same circulation loop as plating cell 102. Therefore, it is impossible to decouple the plating-cell flow rate from the filter flow rate. In the case of electroless cobalt plating, it has been found that quiescent fluid environments are required for particle-free plating. However, since auto-generation of metal particles is practically inevitable in this type of plating environment, flow rates to achieve a quiescent environment result in limited filtration. Limited filtration, therefore, likely leads to an increase in particle load, which further degrades plating solution.
Spray techniques have been suggested for electroless plating. See, for example, U.S. Pat. No. 6,065,424, issued May 23, 2000 to Shacham-Diamand et al. In such techniques, reacting plating solution is applied to a wafer surface as a spray or mist. Typically, the wafer is rotating under the spray or mist, and liquid solution is spun radially outwards. Under such conditions, it is difficult to maintain a sufficiently high and uniform reaction temperature because of the simultaneous cooling of the hot fluid by evaporation of the solvent (e.g., water). Alternatively, heating the backside of the wafer by a heated chuck is possible. Nevertheless, this requires a relatively massive element with sufficient heat capacity to maintain a globally uniform temperature over a standard 200 millimeter (mm) or 300 mm wafer. Also, the face-up base of the heating element/chuck is susceptible to chemical contamination and transfer of that contamination to the wafer backside. Furthermore, backside heating does not solve the problem of non-uniform evaporation and cooling of the bath solvent. On the other hand, a wafer chuck should be capable of spinning at high revolutions per minute (rpm) to enable spin-drying. Splashing of liquid against apparatus walls and misting back onto the product surface can cause contamination of the apparatus and defects on the workpiece. Evaporation and misting of plating solution into the plating space results in substantial loss of the plating solution, and unwanted formation of volatile hazardous chemicals in the effluent.
Maintaining bath concentration, therefore, requires complicated and expensive monitoring and control techniques. U.S. Pat. No. 6,713,122, issued Mar. 30, 2004, to Mayer et al., which is hereby incorporated by reference, solves some aspects of these problems related to process thermal management by decreasing air exposure and by including a recirculation system that decreases the heating load of electroless plating fluid. United States Patent Application Publication No. 2003/0141018, by Stevens et al., published Jul. 31, 2003, teaches an electroless deposition apparatus in which a substrate support holds a substrate wafer in a face-up orientation and an evaporation shield is positioned over the substrate to form a gap that is filled with liquid plating solution.
Wet processing of isolated conductive-metal circuits connected to transistor elements in the presence of light often encounters a number of processing challenges. One problem is the creation of a photo-induced power source when p-n junctions in the base-circuit transistors are exposed to light. Another problem is the completion of a corrosion circuit on the surface being processed between the exposed isolated metal lines and a processing electrolytic solution. The energy of the light photons is converted to electrical energy, creating a reverse bias potential and a corrosion circuit.
Thus, liquid chemical reaction techniques, for example, electroless plating techniques, typically encounter problems such as: difficult or unsuitable control of reaction and process conditions; inability to vary rapidly or dynamically various operating conditions; inability to handle unstable reaction mixtures; accumulation of reaction byproducts; inefficient use of expensive liquid solutions; frequent wafer-handling between process steps; high capital cost of equipment for multi-step processes; and excessive use of valuable clean-room floor space.